1. Field of the Invention
This invention relates to the refining of aluminum. More particularly, it relates to the control of the rotor speed of an aluminum refining system.
2. Description of the Prior Art
Molten aluminum, as derived from most common sources, such as primary metal, scrap and re-melt ingot, usually must be refined, or purified, before being cast into ingots, sheets or bars. This can be accomplished by bubbling an inert sparging gas, e.g., nitrogen or argon, through the aluminum present in molten form in a refining vessel. In some instances, chlorine is also employed. In such refining operations, dissolved hydrogen, non-metallic particles and alkaline and alkaline-earth metals are removed from the molten aluminum. For maximum effectiveness and economical gas usage, the sparging gas is dispersed in the body of molten aluminum in the form of fine bubbles. Such dispersion is advantageously accomplished by the use of a spinning nozzle for the injection of the sparging gas into the molten aluminum.
The refining rate of such a spinning nozzle system can be increased by increasing the flow rate of the sparging gas or gases employed. It is usually necessary, in addition, to increase the nozzle rotating speed to continue the desired formation of small gas bubbles and the dispersing of said bubbles throughout the molten aluminum in the refining zone of the system. Such increase in gas flow and nozzle rotating speed is usually accomplished by increased turbulence on the surface of the molten aluminum. The maximum refining rate for a given refining system is limited by the maximum surface turbulence that can be tolerated in said system.
One very effective manner of using small bubbles and dispersing them in a body of molten aluminum is by the use of a spinning nozzle positioned within said body of molten aluminum in the SNIF.TM. systems of Praxair, Inc. as shown in the Pelton patent, U.S. Pat. No. 4,784,374. In the usual operation of such a system, the spinning nozzle rotor is driven at a constant speed. This is accomplished by driving the rotor with an electric motor controlled by a variable speed electric drive. This allows the rotor speed to be set out at an appropriate value for a particular operation being carried out in the refining system. The rotor speed may be set at different values depending on such variations as process gas flow and system size. Once the speed is set at its optimum value, however, it will remain constant during the refining operation. Short time variations in liquid flow into the rotor will result in variations in load on the rotor and on the motor that drives it. These variations are reflected in variations in motor current as a result of the normal functioning of the control unit used to maintain constant speed. However, average motor current varies smoothly and continuously with the set speed as shown in FIG. 1 of the drawings.
In a high refining capacity system, there is a discontinuity in the speed vs. current relationship as shown in FIG. 2 of the drawings. If the sparging gas flow is commenced and then the drive motor is turned on with the speed set at 450 rpm, the average current will be shown at point A. As the speed is increased by adjusting the speed control dial on the motor drive unit, the average current will increase slowly and continuously as shown by the lower line in FIG. 2, through points E and F until point B at 550 rpm is reached. A further increase in speed setting to 575 rpm will cause the current to increase rapidly to point C on the upper line of FIG. 2. If the speed setting is now decreased, the current will decrease slowly and continuously along values shown on the upper line until point D is reached at 500 rpm. If the speed is lowered to 475 rpm, the current rapidly drops to point E on the lower line.
When operating at any speed-current relations shown on the bottom line of FIG. 2, the bubble pattern obtained in the refining vessel is as shown in FIG. 3. Thus, the bubbles go rapidly upward and produce a rough surface around the spinning nozzle. This is undesirable, both from a refining rate standpoint and with respect to surface roughness.
When operating at any speed-current relation shown on the upper line of FIG. 2, the bubble pattern is as shown in FIG. 4. In this case, the bubbles flow in an outward and somewhat downward direction in the refining vessel, resulting in much better bubble distribution within the refining vessel. This produces a smoother molten metal surface and a higher refining rate compared with operation at point B on the lower line of FIG. 2. However, the surface is not as desirably smooth as it can be. The smoothest surface is obtained by reducing the rotor speed until point D is reached.
After having determined where optimal point D is by the foregoing procedure, it is possible to reach said point D by another route. This is to start rotation at the point D valve, with no gas flow, or with relatively low gas flow, and then to turn on full gas flow after a few seconds of said operation.
By whatever route point D is reached, it has not been possible to operate at this optimal point for very long with speed control in conventional aluminum refining practices. There will occasionally be swings in molten aluminum flow pattern in the refining chamber. As a result, the drive motor current will be found to drop suddenly to point F on the lower line of FIG. 2, with its undesirable bubble pattern. In this circumstance, the desirable operating point D may be re-established only by going through one of the two routes described above.
In practical commercial operations in which the refining system is operated with conventional speed control alone, it is necessary to operate at a higher speed and current than at point D, as, for instance, at point C on FIG. 2. This will provide stable and repeatable operation with the desired bubble pattern, but the surface of the molten aluminum will be much rougher than at the desired point D.
There is a need in the art, therefore, for improvement in the operation of systems for the refining of molten aluminum. Specifically, there is a need for improved means for controlling the rotor speed so as to reach and maintain the optimal point of speed-current during continued refining operations.
It is an object of the invention, therefore, to provide an improved rotor speed control for an aluminum refining system.
It is another object to provide a rotor speed control enabling optimal conditions to be achieved and maintained during refining operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.